Update: This works, but seems to be a bad idea. When I try to do aptitude update it fails with lines like this…

E: Unable to determine file size for fd 7 - fstat (2: No such file or directory)
E: Problem opening /var/lib/apt/lists/ftp.debian.org_debian_dists_jessie_contrib_binary-amd64_Packages
E: The package lists or status file could not be parsed or opened.
E: Unable to determine file size for fd 8 - fstat (2: No such file or directory)
E: Problem opening /var/lib/apt/lists/ftp.debian.org_debian_dists_jessie_contrib_binary-amd64_Packages
E: The package lists or status file could not be parsed or opened.

… which seems to say that 9p has a problem with some operation aptitude needs.